1. Field of the Invention
The present invention relates to a semiconductor device and a data processing system including the same, and more particularly relates to a semiconductor device including a calibration circuit that adjusts an impedance of an output buffer and to a data processing system including the semiconductor device.
2. Description of Related Art
A semiconductor device such as a DRAM (Dynamic Random Access Memory) sometimes includes a calibration circuit that adjusts an impedance of an output buffer (see Japanese Patent Application Laid-open No. 2008-048361, or its patent family member, U.S. Patent Publication 2008/0046212). The calibration circuit is activated by a calibration command issued from a controller.
The calibration command includes a long calibration command (ZQCL) and a short calibration command (ZQCS). The long calibration command (ZQCL) is issued at a time of returning from an initialization period after power activation or from a self-refresh mode, and its calibration period is set relatively long. On the other hand, the short calibration command (ZQCS) is periodically issued during a normal operation, and its calibration period is set relatively short.
As described above, because the short calibration command (ZQCS) is periodically issued from a controller, a timer circuit or the like has to be provided at a controller side, and a control at the controller side becomes complex.
Further, because a read command cannot be issued during a calibration operation, utilization efficiency of a command bus is degraded. In addition, while a resistance value of an external resistor connected to a semiconductor device is referred during a calibration operation, when one external resistor is shared by plural semiconductor devices to reduce the number of elements, calibration operation periods of these semiconductor devices need to be shifted from each other, and thus a control becomes complex.
Therefore, a semiconductor device that automatically performs a calibration has been desired instead of semiconductor devices that issue the short calibration command (ZQCS) from a controller side. However, a calibration operation needs to be periodically performed, and a read operation and a write operation cannot be performed during the calibration operation. Accordingly, when automatically performing a calibration operation at a semiconductor device side, designing needs to take these factors into consideration.